DETROIT – The U.S. government’s road safety agency is accusing Chrysler of moving too slowly to fix some Jeep SUVs in a recall announced more than a year ago.

The National Highway Traffic Safety Administration, in a letter released Wednesday, is asking the automaker why it’s taking so long to fix as many as 2.5 million older Grand Cherokees and Libertys with gas tanks mounted behind the rear axles. The tanks could rupture in rear collisions, leak fuel and cause fires.

The letter is the latest skirmish in a long fight between the automaker and agency over the safety of the SUVs, all built before the 2008 model year. Initially NHTSA wanted the company to recall 2.7 million of them, but Chrysler refused, saying they were as safe as similar vehicles. They eventually worked a deal to recall 1.56 million, with 1.2 million others placed in a campaign to be inspected for hitches. Last year NHTSA said a three-year investigation showed 51 people had died in fiery crashes in Jeeps with gas tanks behind the rear axle.

In the letter, NHTSA said Chrysler will send notices to owners of 1.5 million Grand Cherokees from the 1993 to 1998 model years, and to another 1 million owners of 2002-2007 Libertys.

But the letter says production of the trailer hitches didn’t start until May of this year, and the pace is so slow that it will take Chrysler 4.7 years to get enough hitches if all owners respond to the recall. If only half respond, it will take Chrysler two years to get the parts, the letter said.

“For many owners, a recall remedy deferred by parts availability easily becomes a defect remedy denied,” NHTSA wrote. “The agency has no intention of allowing Chrysler, or any other manufacturer, to delay recall completion to the detriment of safety.”

Chrysler expects to begin repairs on Aug. 1, according to the letter. The company has until July 16 to respond to the agency’s request for information or face up to a $35 million fine, the agency said.

At least part of the delays may be attributed to NHTSA testing the trailer hitch remedy. The agency did crash tests to determine if the hitches would work, and on Jan. 13, it told Chrysler that it had no reservations about the fix.

The company says NHTSA has had full knowledge of its work on the recall and that the company complied with all federal regulations. Chrysler is confident that it will be able to produce enough hitches to satisfy demand, and there could be some sort of misunderstanding with NHTSA, spokesman Mike Palese said.

Chrysler maintains that the SUVs are not defective and says it agreed to the trailer hitches because the matter “has raised public concern.”

But Clarence Ditlow, head of the non-profit Center for Automotive Research, said in a letter to NHTSA Wednesday that the agency should immediately force Chrysler to speed up the recalls. While NHTSA and Chrysler argue, four more people have been killed and two more seriously burned in Jeep fire crashes, according to Ditlow.

Affected customers have been told the recall and service campaigns are coming. The Auburn Hills, Michigan-based company, now part of Fiat Chrysler Automobiles NV, said it will contact them again “when the time is appropriate to schedule service.”

Automakers have 60 days from when they notify NHTSA of a recall to contact vehicle owners. But aside from that, there’s no hard deadline on when repairs have to be finished. NHTSA monitors recalls for at least 1 1/2 years to make sure they are progressing.